Fune 17, 1886 | 
NATURE 
157 
We have received the annual report for 1885 of the Russian 
Geographical Society, which contains short accounts of the 
expeditions of M. Prjevalsky to Central Asia, M. Potanin to 
China, M. Grum-Grzimailo in the sub-Pamir region, MM. 
Wolter and Trusman ; and the usual notices on works for which 
the medals of the Society were awarded. Geographers surely 
will be sorry not to find in this report any notice of the work 
done by the Caucasian and Siberian branches of the Society, 
which usually so greatly increases the value of the annual report 
of the Russian Geographical Society. 
WE are glad to learn from the last Annual Report of the 
Russian Geographical Society that the Appendix to the Ausszan 
Gazetteer, by P. P. Semenoff, is in course of preparation. The 
full edition of the observations at the Polar Stations on Noyaya 
Zemlya and on the Lena; the remarkable collection of maps 
dealing with the delta of the Amu-daria, Baron Kaulbars ; and 
a geological map of the shores of Lake Baikal, are also in 
preparation. 
AT the last meeting of the Paris Geographical Society, Dr. 
Maurel read a paper on his travels in Cochin China and Cam- 
bodia, ona mission from the Minister of Public Instruction. By 
means of aseries of maps representing the Indo-Chinese peninsula 
in the seventh, eleventh, eighteenth, and nineteenth centuries, he 
showed the relative importance at different epochs of each of 
the peoples inhabiting this region. He then gave a general 
account of the country, its geography, climate, population, &c. 
A large collection of ethnographical objects which he had with 
him added much interest to that part of his paper. The young 
Cambodians at present being educated in Paris were present, 
clothed in the national costume. 
THE DETERMINATION OF THE INDEX OF 
REFRACTION OF A FLUID BY MEANS OF 
THE MICROSCOPE 
F the various means adopted hitherto for the determination 
of the refractive index of a fluid, the most usually adopted 
has been that of the hollow prism, telescope, and collimator. 
This method involves (a) the determination of the angle of the 
prism ; (8) the position of minimum deviation ; (y) the use of 
monochromatic light, if errors arising from the different dis- 
persive qualities of the substances are to be avoided. These 
preliminaries render the labour of determining the index a very 
difficult task, and the observer will scarcely expect to accomplish 
more than one observation at a sitting. 
Cleaning the prism is not the least of the troubles, and when 
we add to them the fact that many liquids are so opaque that 
sufficient light can scarcely be passed through them for the 
observation, it is not surprising that so few have been found to 
possess the courage necessary for attacking the problem. The 
writer having had occasion for frequent determination of the 
index of refraction, has found the use of the microscope far sur- 
passes the usual method in giving results of the greatest delicacy 
combined with a minimum of cost and of time. 
Starting with the well-known fact, that an object viewed 
through a medium whose refractive index is different from that 
of air will occupy a different position from its image, or in the 
language of the text-books, v = # + £\ where 7 determines the 
Kb 
position of the geometrical focus of a pencil after direct refraction 
through a plate whose thickness is ¢, the writer was led to adopt 
the following plan. 
On an ordinary ‘‘slip” as used for mounting preparations for 
the microscope a delicate mark is made with a writing diamond, 
A large but very thin ‘‘cover-glass ” is cut in half, and its pieces 
cemented to the ‘‘slip” on either side of the mark, leaving a 
space of about one-eighth of an inch ; then, resting on these sup- 
ports, and bridging over the intervening space, is placed a small 
but very thin ‘‘cover-glass,”’ and a drop of the fluid to be 
examined is run under this. 
The fine mark made on the ‘‘slip” is now viewed through 
this with the microscope, using as high a power as possible, for 
the higher the objective the more delicate will be its focal adjust- 
ment; when the object is in focus the position of the ‘fine 
adjustment” must be read off. The microscope must then be 
left, and the slip removed for the examination of any other fluid. 
The top cover-glass is lifted off, the slip cleaned, the same cover- 
glass replaced, and a drop of a different fluid run under. Re- 
placing now the slip upon the stage, and looking for the mark 
which was previously in focus, it will be found that an altera- 
tion of the fine adjustment is necessary to bring it into focus. 
If the medium is of lower refractive index, the objective 
will have to be lowered, and conversely. Thus a rapid com- 
parison of the relative refractive indexes of two media may 
easily be made. 
But not only can the relative refractive powers of different 
bodies be thus obtained ; the absolute numerical values may 
with the greatest accuracy be determined. For this it is essen- 
tial that the fine adjustment screw should have accurate micro- 
meter divisions, and this is usually the case now that immersion 
objectives are in common use. Two fluids must be selected 
whose refractive indexes present a wide difference, say oil of 
cassia and water ; focus the mark, first viewed through water, 
secondly viewed through oil of cassia, and read off the number 
of divisions the screw has been turned through in the alteration 
of the focus. The refractive indexes of oil of cassia and water 
being known from the tables, a numerical value will by the 
formula be obtained for each division of the screw-head, and 
thus the absolute numerical index of any medium easily be 
determined. 
By this simple and inexpensive method the writer has ob- 
tained from fifteen to twenty absolute indexes in a sitting of 
an hour’s duration. 
The importance of obtaining suitable media of high refractive 
index for mounting objects to be viewed with very high powers 
cannot be overestimated, for not only is a wider cone of light 
thus brought to bear upon the object, but its image is advanced, 
so that a greater working distance is obtained between the front 
lens of the objective and the coyer-glass. 
GoRDON THOMPSON 
St. Charles's College, Noiting Hill 
UNIVERSITY AND EDUCATIONAL 
INTELLIGENCE 
CAMBRIDGE.—The twentieth annual report of the Museums 
and Lecture-Rooms Syndicate, lately issued, recounts continuous 
progress in many scientific departments. The number of 
students attending demonstrations in the Cavendish Laboratory 
reached 100 last winter, and during the year twelve persons 
have done original work in the Laboratory. 
The Plumian Professor (Mr. G. H. Darwin) introduced a new 
feature last summer by giving a course of lectures in the Long 
Vacation, and the attendance (thirteen) was encouraging. Few 
students attend the Plumian Professor’s advanced lectures on 
the orbits and perturbations of planets. 
In mechanism Prof. Stuart reports that the temporary museum 
and lecture-room has become very insufficient. 
In chemistry there has been a considerable increase of 
students in advanced classes and special departments. The new 
laboratory is now being vigorously advanced. The classes in 
mineralogy maintain an average of sixteen students. The 
acquisition of 250 specimens from Mr. Field’s collection has 
added some minerals previously unrepresented, and has im- 
proved the collection considerably for students’ use. 
In geology Prof. Hughes regrets the disadvantages of his 
present accommodation for teaching and lecturing, and finds 
the specimens of value are lost to the Museum because of its 
inadequate means of displaying them. A valuable collection of 
Cretaceous Cambridge fossils, many of them type-specimens, 
has been presented by Mr. James Carter of Cambridge. 
Mr. Marr, Fellow of St. John’s College, is engaged upon 
the arrangemext of the Foreizn and British Cambrian fossils, of 
which it will be desirable soon to publish a new catalogue. The 
petrological series has been rearranged, and also the collection of 
microscope slides. ‘lhe Upper Jurassic fossils have been largely 
added to and rearranged. Many interesting additions to the 
museum are chronicled in the report. It shows how largely the 
Museum gains from the interest of present and former students at 
Cambridge. 
Prof. Babington has been chiefly occupied with the study of 
different parts of the Herbarium—especially the magnificent col- 
lection of European Rubi—and the identification of piants sent 
by botanists from a distance. Dr. Vines’s students haye num- 
bered nearly sixty, and the Botanical Laboratory is incon- 
yeniently crowded, The commencement of a botanical museum 
has been made by Messrs. Potter and Gardiner, with the object 
